SIMULTANEOUS BUT INDEPENDENT ACTIVATION OF ADENYLATE-CYCLASE AND GLYCOSYLPHOSPHATIDYLINOSITOL-PHOSPHOLIPASE-C UNDER STRESS CONDITION IN TRYPANOSOMA-BRUCEI

Previous observations suggested a concomitant relationship between the release of the variant surface glycoprotein (VSG) and the activation of adenylate cyclase in the bloodstream form of the parasitic protozoa n Trypanosoma brucei, In order to evaluate this hypothesis, adenylate cyclase activity was measured in live trypanosomes subjected to differ ent treatments known to induce the shedding of the VSG coat, namely lo w pH and trypsin digestion, In both cases adenylate cyclase activation occurred in parallel with the release of the VSG, The latter was foun d to be mediated by the glycosylphosphatidylinositol-specific phosphol ipase C that cleaves the glycosylphosphatidylinositol anchor of the pr otein (VSG lipase). Furthermore, both adenylate cyclase and VSG releas e were activated by the incubation of trypanosomes with specific inhib itors of protein kinase C, suggesting a repressive role for protein ki nase C on both VSG lipase and adenylate cyclase activities. Significan tly, in mutant trypanosomes lacking VSG lipase, adenylate cyclase was activated under conditions where VSG release did not occur. Moreover, VSG release was also found to occur in the absence of activation of th e cyclase, as observed in the presence of low concentration of the thi ol modifying reagent p-chloromercuriphenylsulfonic acid. These observa tions provide the first demonstration that release of the VSG in respo nse to cellular stress is mediated by the VSG lipase and that while bo th release of the VSG and activation of adenylate cyclase occur in res ponse to the same stimuli they are not obligatorily coupled.